I was capturing mines along the way, but then the computer would recapture them. My goal was to complete the mission quickly with maximum hero development, so it was problematic to deal with the mines. As for troops, I would only reinforce myself if I was lucky and, while passing by a castle (or capturing an enemy castle), there were units available to hire (or at least the next day). Otherwise, I would just run further. Therefore, the enemy outnumbered me considerably. But the issue isn't about numbers...
By the way, I haven't played version 1.4; I still have version 1.0 installed...
The computer tells me the order in which to capture cities. I go where the most enemies are coming from, because they have moved troops away from there. This means it will be easier to capture the city. That's why Railag went to the portal near the first city.
I started playing with version 1.3. I played for a week, and then patch 1.4 was released. Before that, I hadn't played any of the Heroes games. In 1.4, they made it easier – now troops accumulate in dwellings. That's why I capture all enemy dwellings, even when I don't plan to buy troops there. I do it solely to destroy all the creatures. Otherwise, some enemy might wander in and buy up all the creatures that have accumulated over many weeks.